A sealing device of this kind has been known in the related art and disclosed in JP-A-9-42463, for example, as shown in FIG. 7.
This sealing device 100 is mounted in the annular clearance between a housing 102 and a shaft 103, i.e., two members to be assembled to rotate relative to each other, so that it seals up that annular clearance thereby to prevent a sealed object such as oil from leaking out.
The sealing device 100 is constructed to include: an annular seal body 105 to be fitted on the inner circumference of a shaft bore formed in the housing 102; and a reinforcing ring 104 to be fitted in the seal body 105 for giving it rigidity.
The seal body 105 is provided with a seal lip 107, which extends radially inward from an atmospheric side A toward an sealed object side O. Moreover, the seal lip 107 is provided at its leading end portion with a lip leading end 110 having a wedge-shaped section. This lip leading end 110 has a predetermined interference with the circumference of the shaft 103 so that it is slidably brought into sealing contact with the circumference of the shaft 103 by its elastic restoring force thereby to prevent the sealed object from leaking out.
On an atmospheric side slope 111 of the seal lip 107, on the other hand, there are formed screw ridges 112. Each of these screw ridges 112 is shaped by combining a bilge-shaped portion 112a, which is larger in height and width toward the axially intermediate portion, and a straight portion 102b having substantially constant height and width. The straight portion 102b merges into the end portion of the bilge-shaped portion 112a on the side of the lip leading end 110 and extends to the lip leading end 110.
By thus providing the screw ridge 112 in which the bilge-shaped portion 112a and the straight portion 112b are continuously combined with the atmospheric side slope 111 of the seal lip 107, the initial sealing properties are retained by the straight portion 112b, and the deterioration in the sealing properties of the seal lip 107 while the wear is proceeding is prevented by the bilge-shaped portion 112a. 
However, the case of the related art thus far described has the following problems.
Generally, the sealing device made of elastomer is prepared by pouring elastic material into a molding die and by vulcanizing and shaping it. In order to form the screw ridges on the seal lip, therefore, recesses conforming to the screw shape have to be formed in advance in the molding die.
In order to obtain the screw ridge 112 having the aforementioned two different shapes combined, two steps shown in FIGS. 8A and 8B are required for manufacturing the molding die. At first, a cutter 114 having a straight blade edge is pushed onto a molding die 116, as shown in FIG. 8A, to form a recess (depression) 117 corresponding to the straight portion. Next, as shown in FIG. 8B a cutter 115 having an arcuate blade edge is pushed onto the recess 117 to form a recess 118 corresponding to the bilge-shaped portion. Thus, the sealing device of the conventional construction invites complexity and complicatedness in the processing step of the molding die 116 and is demanded for a high working precision.
On the other hand, the lip leading end 110 of the seal lip 107 of the related art is a sharp edge. Therefore, the fastening pressure (or the facial pressure) at the initial action time is raised to cause a problem that the proceeding of the wear of the lip leading end 110 is accelerated.
In the wedge-shaped lip of the related art, moreover, it is necessary that the unnecessary portion of the lip leading end is cut off with a knife after the vulcanized and shaped seal body was taken out from the molding die 116. This necessity complicates the working process of the sealing device itself and raises the cost. Another problem is that this knife-cutting step easily causes dispersion in the cutting position.
The invention has been conceived in view of the background thus far described, and has an object to provide a sealing device which has an easy manufacturing and a slow proceeding of wear so that satisfactory sealing properties can be exhibited initially and endurably for a long time.